1. Field of the Invention
The present invention relates to a method for converting oxygenate, such as methanol and/or dimethyl ether, to olefins in a reactor over a framework gallium-containing medium pore molecular sieve catalyst such as ZSM-5 or ZSM-11 wherein oxygenate can be co-fed with aromatics. The method is especially useful for increasing ethylene/propylene product ratio.
2. Description of the Background Art
A remarkable growth in the production of synthetic fibers, plastics and rubber has taken place in recent decades. This growth, to a very large extent, has been supported and encouraged by an expanding supply of inexpensive petrochemical raw materials such as ethylene, propylene, as well as four and five carbon olefins. Along with this growth has been an increasing demand for alkylate, made by reacting olefins with isobutane, for use as a high octane gasoline component.
Burgeoning demand for olefins, particularly ethylene, propylene and butenes, has of course led to periods of shortage, causing substantial price increases in the feedstocks to various commercialized technologies. These feedstocks are largely C2 to C4 paraffins co-produced with natural gas and/or paraffinic straight run naphtha. Such feedstocks can be substantially more expensive than methane, making it desirable to provide efficient means for converting methane to olefins.
Conversion of methane to methanol followed by conversion of methanol to light olefins is among the most economic routes to make light olefins from methane. In this respect, it is known that methanol or dimethyl ether can be catalytically converted to olefin-containing hydrocarbon mixtures by contacting under certain conditions with particular types of crystalline zeolite materials. U.S. Pat. Nos. 4,025,575 and 4,038,889, for example, both disclose processes whereby methanol and/or dimethyl ether can be converted to an olefin-containing product over a Constraint Index 1-12 zeolite catalyst, particularly ZSM-5. In fact, ZSM-5 converts methanol and/or dimethyl ether to hydrocarbons containing a relatively high concentration of light olefins with prolonged catalyst lifetime before catalyst regeneration becomes necessary. U.S. Pat. No. 4,311,865 teaches the use of the medium pore zeolite, ZSM-5 (approximately 5.5 Angstroms) pore size, which is ion-exchanged with cobalt, and then calcined to produce a catalyst, and has been used to convert methanol to hydrocarbons (including olefins). This process uses ion-exchange to add the metal to the medium pore molecular sieve. Despite the durability of these medium pore size catalysts, they exhibit a low selectivity for ethylene when converting oxygenates. For example, HZSM-5 can exhibit ethylene selectivity of less than 5%.
It has also been reported that other types of zeolite catalysts can be used to convert methanol and/or dimethyl ether to olefin-containing hydrocarbons products containing higher proportions of light olefins than previously obtained with ZSM-5. For example, U.S. Pat. No. 4,079,095 discloses that zeolites of the erionite-offretite-chabazite type, and especially ZSM-34, can promote conversion of methanol and/or methyl ether to products comprising a major amount of ethylene and propylene. However, while erionite-offretite-chabazite type catalysts are highly selective to light olefins production, such smaller pore zeolites tend to age rapidly in comparison to ZSM-5 when used for methanol/dimethyl ether conversion, perhaps owing to the rapid buildup of coke inside the zeolite cages, which blocks the accessibility of methanol feed to the acid sites contained therein. Small pore catalysts such as SAPO-34 have been used to convert methanol to olefins, as described in an article by T. Inui, “Structure-Reactivity Relationships in Methanol to Olefins Conversion in Various Microporous Crystalline Catalysts, Structure-Activity and Selectivity Relationships in Heterogeneous Catalysts”, pages 233-42, Elsevier Science Publishers, B. V., Amsterdam (1991). U.S. Pat. No. 5,962,762 to Sun et al. discloses a method for converting starting material to olefins comprising contacting the starting material with a small pore molecular sieve catalyst such as SAPO-34 under effective conditions to produce olefins, wherein the molecular sieve has been modified after synthesis by incorporation of a transition metal ion using a transition metal compound, wherein the transition metal ion is selected from Groups VIB, VIIB, and VII.
T. Mole, G. Bett, and D. J. Seddon, Journal of Catalysis 84, 435 (1983), disclose that the presence of aromatic compounds can accelerate the zeolite-catalyzed conversion of methanol to hydrocarbons. The article reports ethylene yields of 5-22% when methanol is catalytically converted in the presence of benzene or toluene over ZSM-5 at sub-atmospheric pressure, 279° to 350° C., and 100% methanol conversion. U.S. Pat. No 4,499,314 ('314 Patent) discloses that the addition of various promoters, including aromatic compounds, such as toluene, accelerate the conversion of methanol to hydrocarbons over zeolites, such as ZSM-5, which have a pore size sufficient to permit sorption and diffusion of the promoter. In particular, the '314 Patent teaches that the increased conversion resulting from the addition of the promoter allows the use of lower severity conditions, particularly lower temperatures, which increase the yield of lower olefins (column 4, lines 17-22). Thus, in Example 1 of the patent the addition of toluene as a promoter reduces the temperature required to achieve full methanol conversion from 295° C. to 288° C. while increasing the ethylene yield from 11 wt. % to 18 wt. %. In the Examples of the '314 patent the methanol feedstock is diluted with water and nitrogen such that the methanol partial pressure is less than 2 psia (14 kPa). U.S. Pat. Nos. 4,677,242 and 4,752,651 disclose the conversion of methanol to C2 to C4 olefins over various silicoaluminophosphates such as SAPO-34, and “non-zeolitic molecular sieves” (such as metal aluminophosphates), and teach that the addition of diluents, such as aromatic materials, having a kinetic diameter greater than the pore size of the molecular sieve, increases the ethylene to propylene ratio in the product.
Conversion of ethane to aromatics over GaZSM-5 is disclosed in U.S. Pat. No. 4,350,835. U.S. Pat. No. 4,605,805 discloses a catalyst for olefin/paraffin conversion that employs ZSM-5 having a framework wherein gallium is substituted for boron or iron. U.S. Pat. No. 5,023,391 discloses a process for the direct partial oxidation of methane with oxygen, whereby organic compounds comprising higher hydrocarbons are produced. The catalyst used in this reaction is a GaZSM-5 catalyst. This catalyst may be prepared by ion exchanging or impregnating a ZSM-5 catalyst with a suitable gallium salt such as gallium nitrate.
U.S. Pat. No. 5,981,817 discloses a xylene isomerization process conducted in the presence of hydrogen and at pressures in excess of 75 psig (620 kPa) over a ZSM-5 catalyst containing about 0.1 to 5 wt. % of at least one metal selected from the group consisting of zinc, copper, silver and gallium. The catalyst may be a zeolite bound zeolite prepared in accordance with U.S. Pat. No. 5,460,796.
All of the foregoing references are incorporated herein by reference.
In spite of the existence of methanol conversion processes utilizing a variety of zeolite catalysts and process conditions, there is a continuing need to develop new procedures suitable to convert an organic feed comprising oxygenates, such as methanol or dimethyl ether, selectively to light olefin products and, in particular, ethylene.